Method for refining easily polymerizable matter-containing substance and apparatus therefor

ABSTRACT

A method for refining an easily polymerizable matter while preventing an easily polymerizable matter-containing substance from being polymerized and an apparatus for use in the method are disclosed. The method and apparatus for refining and separating the easily polymerizable matter-containing substance by the use of a distillation column provided with a heat exchanger set the linear velocity of a vapor in a connecting line intervening between the distillation column and the heat exchanger at a level of not less than 5 m per second and the retention time of the vapor at a level of not more than 3 seconds.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method for refining an easilypolymerizable matter-containing substance and an apparatus therefor.More particularly, it relates to a method for refining (meth) acrylicacid and/or a (meth) acrylic ester while preventing the acid and/or theester from being polymerized particularly in the vapor phase and anapparatus to be used therefor.

[0003] 2. Description of the Related Art

[0004] Such an easily polymerizable matter as impurities-containing(meth)acrylic acid has been heretofore refined as follows.

[0005] In a distillation column with a condenser and a re-boiler, thecondenser is connected to the top of the distillation column via a(inter) connecting line and the re-boiler is positioned near the lowerpart of the distillation column via a (inter)connecting line. Here, thefeed material such as the impurities-containing (meth)acrylic acid issupplied to the distillation column and distilled therein. The vapordistilled from (meth)acrylic acid which has entered the condenser viathe connecting line is condensed and apart thereof is returned to thedistillation column and the rest thereof is recovered as distillate D orsubjected further to a subsequent treatment. Meanwhile, the liquidcontaining (meth)acrylic acid through the bottom of the distillationcolumn is supplied to the re-boiler and reboiled therein, with theresult that the vapor containing (meth) acrylic acid will be returnedvia the connecting line to the distillation column.

[0006] It is well known that (meth)acrylic acid and/or a (meth)acrylicester is so easily polymerized very often during the process ofproduction such as refinement and that the apparatus of productioncompellingly stopped. As a measure, the practice of adding apolymerization inhibitor such as hydroquinone or phenothiazine duringthe production of (meth)acrylic acid and/or (meth)acrylic ester therebypreventing polymerization in the liquid phase of the purificationprocess has found popular recognition.

[0007] U.S. Pat. No. 3,988,213 discloses a method for preventing thevapor of a vinyl compound from condensing or polymerizing by providing adistillation column with a jacket.

[0008] Further, U.S. Pat. No. 3,717,553 discloses a distillation columnwhich, by having an opening formed near the wall of a tray tower andconsequently allowed to have the wall of its own wetted, is enabled toprevent a vinyl monomer from polymerizing.

[0009] Perfect prevention of the polymerization that occurs during thedistillation of an easily polymerizable matter or during thecondensation of a vapor or the re-condensation of the vapor in a vaporphase has not yet been accomplished.

SUMMARY OF THE INVENTION

[0010] An object of the invention is to provide a method for refining aneasily polymerizable matter-containing substance such as (meth)acrylicacid and/or a (meth)acrylic ester, which attains the refinementefficiently by efficiently preventing the matter under treatment frompolymerizing in a vapor phase part of a connecting line interposedbetween a distillation column and a heat exchanger, and an apparatus tobe used therefor.

[0011] The present inventors have pursued a study and consequently foundthat in the distillation of (meth)acrylic acid and/or a (meth)acrylicester, the compound being distilled is prevented from polymerizing bycausing a vertical shell-and-tube heat exchanger to be supported by thedistillation column and by branching a vapor line interconnecting themfrom the direct shell part of the distillation column or setting acondenser directly on the top of the distillation column or setting are-boiler in the bottom of the distillation thereby decreasing the vaporphase volume thereof. This invention has been achieved as a result.

[0012] Specifically, according to the first aspect of this invention itcan provide a method for refining and separating an easily polymerizablematter-containing substance by using a distillation column furnishedwith a heat exchanger, which method is characterized by setting thelinear velocity of the vapor in a connecting line interconnecting thedistillation column and the heat exchanger at a level of not less than 5m per second and the retention time of the vapor in the connecting lineat a level of not more than 3 seconds.

[0013] According to the second aspect of this invention, it can providean apparatus for refining an easily polymerizable matter-containingsubstance by using a distillation column furnished with a heatexchanger, which apparatus is characterized by using a condenser as theheat exchanger and having the condenser directly set on the top of thedistillation column.

[0014] Then, according to the third aspect of this invention, it canprovide-an apparatus for refining an easily polymerizablematter-containing substance by using a distillation column furnishedwith a heat exchanger, which apparatus is characterized by using are-boiler as the heat exchanger and the re-boiler being disposeddirectly inside the distillation column in the lower part thereofwherein a vapor guide provided above the heating tubes for the re-boileris opened into the vapor phase part of the column and a down-take isfurnished outside the heating tubes.

[0015] As described above, this invention, in refining and separating aneasily polymerizable matter-containing substance by using a distillationcolumn furnished with a heat exchanger, is capable of effectivelypreventing the vapor of the easily polymerizable matter-containingsubstance from polymerizing by setting the linear velocity of the vaporin a (inter)connecting line interconnecting the distillation column andthe heat exchanger at a level of not less than 5 m per second and theretention time of the vapor in the connecting line at a level of notmore than 3 seconds.

[0016] Further, by using the apparatus of this invention for refining aneasily polymerizable matter-containing substance, the vapor of theeasily polymerizable matter-containing substance can be effectivelyprevented from polymerizing.

[0017] The above and other objects, features, and advantages of thepresent invention will become clear from the following description ofthe preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is an explanatory diagram illustrating one example of thedistillation column that is furnished with heat exchangers and used inthis invention;

[0019]FIG. 2 is a sectional view illustrating one example of thedistillation column in which a re-boiler is furnished inside the lowerpart thereof, a condenser furnished in the upper part thereof and usedin this invention;

[0020]FIG. 3 is a fragmentary sectional view of another embodiment ofparts of FIG. 2 showing a (inter) connecting line between thedistillation column and the condenser;

[0021]FIG. 4 is a fragmentary front view of another embodiment of partsof FIG. 2 showing a liquid line between the distillation column and thecondenser;

[0022]FIG. 5 is a fragmentary front view of another embodiment of partsof FIG. 2 showing a down-take;

[0023]FIG. 6 is a sectional view illustrating one example of thedistillation column that is furnished with a condenser directly in thetop part of the distillation column, with a re-boiler in the lower partthereof and used in this invention; and

[0024]FIG. 7 is a fragmentary sectional view of another embodiment ofparts of FIG. 6 showing a (inter) connecting line between thedistillation column and the re-boiler.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The easily polymerizable matter for use in this invention has noparticular restriction but is only required to be a matter which easilypolymerizes. Polymerizable vinyl compounds may be cited as examples. Theterm “polymerizing vinyl compounds” as used herein may include(meth)acrolein; (meth)acrylic acid; hydroxyethyl (meth) acrylate,hydroxypropyl (meth)acrylate, glycidyl (meth)acrylate, methyl(meth)acrylate, and butyl (meth)acrylate, ethyl (meth)acrylate,2-ethylhexyl (meth)acrylate, N,N-dimethyl amino ethyl (meth)acrylate,and mixtures thereof.

[0026] The term “easily polymerizable matter-containing substance” asused in this invention refers to a mixture, preferably a mixed solution,which comprises the easily polymerizable matter mentioned above and aby-product produced during the synthesis or formation of the easilypolymerizable matter. In the case of acrylic acid or an acrylic ester,for example, this term refers to a solution of acrylic acid and anacrylic ester which contains by-products such as acetic acid, propionicacid, acrolein, maleic acid, water, and formalin which arise when theacrylic acid is obtained by the reaction of catalytic gas phaseoxidation (JP-A-09-227,445 refers, for example). Then, in the case ofmethacrylic acid and a methacrylic ester, for example, the term refersto a solution of methacrylic acid and a methacrylic ester which containsby-products such as methacrolein, acrylic acid, and acetic acid whichoccur when the methacrylic acid is obtained by the reaction of catalyticgas phase oxidation (JP-B-03-24,459 refers, for example).

[0027] The distillation column to be used in this invention is notparticularly limited but capable of effecting distillation of the easilypolymerizable matter-containing substance. Examples of the distillationcolumn may include mono-stage towers and multi-stage towers or columnssuch as a packed tower, a plate tower (tray tower), a wetting walltower, and a spray tower. Among other types of distillation columnmentioned above, the plate tower (multi-stage tray column) provesparticularly advantageous from the viewpoint of prevention ofpolymerization and tower efficiency. The conditions for the distillationare not particularly limited but are only required to fit the purpose ofdistilling the easily polymerizable matter-containing substance.Properly, the column top temperature may be in the range of above 40° C.from the view point of using a cooling water at the top of the column,on the other hand, may be in the range of below 100° C. from the viewpoint of handling the easily polymerizable matter. The column bottomtemperature may be in the range of below 150° C., preferably 60 to 100°C. from the view point of polymerization of the easily polymerizablematter, thus according to the above, temperatures the column pressuremay be determined and the ref lux ratio be in the range of 1.2 to 2.0times based on the minimum reflux ratio.

[0028] The shell-and-tube heat exchanger contemplated by this inventionis used for the purpose of cooling the vapor emanating from the top ofthe distillation column or generating the vapor from the bottomsthereof. An ordinary shell-and-tube heat exchanger can be used for thispurpose in the present invention. The term “shell-and-tube heatexchanger” as used in this invention means a condenser and a re-boiler.Normally, the shell-and-tube heat exchanger has a construction such thata bundle of a multiplicity of slender heat transfer tubes of small wallthickness is disposed in a cylindrical shell and effects exchange ofheat between the fluids contacting the inner and outer sides of the heattransfer tubes, each fluid is introduced from an inlet, respectively.Generally, it is provided with a baffle plate for the purpose ofenabling the shell side fluid to flow most effectively relative to theheat transfer tubes, exalting the efficiency of heat transfer, andretaining the heat transfer tubes fast in position. The heat exchangerof the type that is a vertical posture and passes the process fluidinside the tubes proves particularly advantageous, among other types ofheat exchangers, from the viewpoint of preventing polymerization. Theoperating conditions for the condenser correspond to those of thecolumn. Also, the operating conditions for the re-boiler correspond tothose of the column.

[0029] Properly, in this invention, the linear velocity of the vapor inthe connecting line between the distillation column and the heatexchanger or the equivalent of connecting line is set at a level of notless than 5 m per second, preferably in the range of 5-60 m per second,and more especially in the range of 20-60 m per second, and at the sametime the retention time of the vapor in the connecting line is set at alevel of not more than 3 (excluding 0) seconds, preferably not more than2 (excluding 0) seconds. The term “vapor” as used herein means an easilypolymerizable matter as vapor, in other words an easily polymerizablematter is vaporized and the vapor may contain a gas such as nitrogengas. The linear velocity of the vapor is measured in a condition thatexists during the course of the stationary operation of the distillationcolumn, excluding the initial and terminal step of the distillation. Thelinear velocity of the vapor refers, when the distillation column andthe heat exchanger are interconnected with a connecting line, to themagnitude that exists between the inlet and the outlet of thisconnecting line. Naturally, the distance to the joint of thedistillation column to the connecting line and the distance to the jointof the heat exchanger to the connecting line are decreased to thefullest possible extent on the condition that the disassemble of thejoint is attained as needed by this invention. If the linear velocity ofthe vapor is less than 5 m per second, the adhesion of the easilypolymerizable matter-containing substance to the wall of line willbecome visually discernible. Conversely, if the linear velocity of thevapor exceeds 60 m per second, the excess will be at a disadvantage inenlarging the pressure loss and suffering the rise of temperature tobecome prominent. If the retention time of the vapor exceeds 3 seconds,the excess will be at a disadvantage in suffering the adhesion of theeasily polymerizable matter-containing substance to the wall of thepipes to become visually discernible.

[0030] The linear velocity contemplated by this invention is determinedaccording to the following formula:

[0031] Linear velocity (m/s)=flow of the vapor(kg/s)/(vapor density(kg/m³)×area of the connecting line(m ²)).

[0032] The retention time contemplated by this invention is determinedaccording to the following formula:

Retention time=G/(H×I)

[0033] wherein G stands for the inner volume of the connecting line(m³), H for the linear velocity of the vapor in the connecting line(m/s), and I for the cross sectional area of the connecting line (m²).

[0034] In this invention, the distance over which the vapor passesbetween the distillation column and the heat exchanger is preferred tobe substantially 0. The expression “substantially 0” as used hereinrefers to disposing the heat exchanger on the distillation column,namely avoiding use of such other member or part as a (inter) connectingline between the distillation column and the heat exchanger. When theheat exchanger is serving as a condenser, this expression depicts thefact that this condenser is disposed-directly on the top part of thedistillation column. When the heat exchanger is serving as a re-boiler,the expression depicts the fact that the re-boiler is disposed directlyin the lower part, preferably in the lowest part, of the straight shellof the distillation column.

[0035] The re-boiler that is disposed directly inside the column bymeans of flanges in the lower part thereof is composed of

[0036] a cylindrical vessel provided on the lateral side thereof with aninlet and an outlet for a heating medium and tightly sealed in the upperand lower part thereof,

[0037] a plurality of heat transfer tubes disposed between the upper andlower sides of the cylindrical vessel and provided respectively thereinwith a vertically piercing hole,

[0038] a vapor guide disposed above the cylindrical vessel and having avapor vent positioned approximately in the central part of thedistillation column,

[0039] a loose lid disposed on or above the vapor vent, and

[0040] a down-take disposed in such a manner outside the heating tube asto join the upper and lower parts of the cylindrical vessel.

[0041] In addition, in order to heighten the effect of preventingpolymerization as another method it is preferable that the innerdiameter, against the flow direction of the vapor, of the connectingline, from the column to the tube sheet of the heat exchanger, tosubstantially coincide with that of the heat exchanger shell in the viewpoint of good distribution of the vapor and preventing the channelingthereof. In this sentence, the term “substantially” means that thevelocity of vapor based on the difference of the sectional areas doesnot affect the prevention of polymerization. Of course, in this case itis essential to fulfil the conditions of the linear velocity of thevapor and the retention time.

[0042] Now, this invention will be described below with reference to thedrawings.

[0043]FIG. 1 is a front view of a distillation column equipped with acondenser and a re-boiler. The condenser is supported as connected tothe upper part of the straight shell of the distillation column so as todecrease the vapor phase volume of the connecting line and shorten theretention time of the substance under treatment in the connecting line.The re-boiler is supported as connected to the lower part of thestraight shell so as to decrease the vapor phase volume of theconnecting line. Here, a vertical heat exchanger may be used as thecondenser, a vertical heat exchanger as the re-boiler, and a plate toweras the distillation column as popularly accepted. With reference to FIG.1, the vapor phase volume of a connecting line 9 is decreased to thefullest possible extent by minimizing the distance between the outlet ofa distillation column 1 and the inlet of a condenser 5. For thispurpose, the connecting line 9 to the condenser (shell-and-tube heatexchanger) 5 is drawn out of the straight shell near the top of thedistillation column 1 and supported by the distillation column 1 (FIG. 1omits illustrating a support for fastening the condenser to thedistillation column). Though the connecting member or (inter) connectingline is not particularly limited on account of shape (such factors asradius of curvature, angle of bend, and angle of diversion), it ispreferred to shorten the distance of connection with a view toheightening the effect of connection. To be specific, it is proper toconnect the distillation column and the condenser with a 90°(miter)bend/elbow joint or a short pipe/reducer. The connection of theconnecting line to the distillation column or to the condenser can beattained by any of the known methods such as, for example, welding orbolt-and-nut fastening through flanges. An easily polymerizablematter-containing substance F such as (meth)acrylic acid containingimpurities is supplied to the distillation column 1 to effectdistillation of (meth) acrylic acid therein. Since the length of theconnecting line 9 from the top of the distillation column 1 to thecondenser 5 is smaller than heretofore usual, the polymerization of thevapor of the easily polymerizable matter such as (meth)acrylic acid inthe connecting line 9 can be decreased. It is naturally necessary thatthe linear velocity of the vapor in the connecting line 9 be not lessthan 5 m per second and the retention time of the vapor therein be notmore than 3 seconds. The liquid as of the (meth)acrylic acid which hasbeen indirectly cooled with a known coolant such as water or a coolingliquid in the condenser 5 is handled as a ref lux liquid R or adistillate D.

[0044] A re-boiler 7 is disposed in the lower lateral part of thedistillation column 1 using a (inter) connecting line 6. The vapor phasevolume of the connecting line 6 is decreased to the fullest possibleextent by minimizing the distance between the outlet of the re-boiler 7and the inlet of the distillation column 1. For this purpose, the outletof the re-boiler 7 is connected to the straight shell near the bottom ofthe distillation column 1 (FIG. 1 omits illustrating a support forfastening the re-boiler to the distillation column). Though theconnecting member or connecting line is not particularly limited onaccount of shape (such factors as radius of curvature, angle of bend,and angle of diversion), it is preferred to shorten the distance ofconnection with a view to heightening the effect of connection. To bespecific, it is proper to connect the distillation column and there-boiler with a 90° (miter) bend/elbow joint or a short pipe/reducer.The liquid of the easily polymerizable matter as (meth)acrylic acidwhich is in the bottom part of the distillation column 1 flows throughthe connecting line 2 and enters the re-boiler 7. The vapor from theliquid re-boiled in the re-boiler 7 passes through the connecting line 6and enters the vapor phase part of the distillation column 1. In theconnecting line 6, the polymerization of the vapor is prevented orallayed because the linear velocity of the vapor is not less than 5 mper second and the retention time thereof is not more than 3 seconds.Then, the bottom stream from the re-boiler 7 is discharged via theconnecting line 2 (B). The connection of the connecting line to thedistillation column or to the re-boiler can be accomplished by any ofthe known methods such as welding or bolt-and-nut fastening throughflanges. Generally, the distillation column mentioned above is supportedby any of the known devices such as a structure (not shown).

[0045]FIG. 2 is a sectional view illustrating a distillation columnfurnished with a condenser and a re-boiler, both condenser and re-boilerhaving a plurality of tubes therein. The condenser is supported asconnected to the upper part of the straight shell of the distillationcolumn to decrease the vapor phase volume of the connecting line andshorten the retention time of the matter under treatment, and a naturalcirculation type re-boiler is further disposed directly in the lowerpart of the straight shell of the distillation column. Here, thecondenser and the distillation column are not particularly limited butallowed to use well-known devices such as a vertical heat exchanger anda plate tower. In the diagram, like members or parts found in FIG. 1will be represented by like reference numerals. With reference to FIG.2, the vapor phase volume of the connecting line 9 is decreased to thefullest possible extent by minimizing the distance from the outlet ofthe distillation column 1 to the inlet of the condenser 5. For thispurpose, the connecting line 9 to the condenser (shell-and-tube heatexchanger) 5 is drawn out of the straight shell near the top of thedistillation column 1 and supported by the distillation column 1 (FIG. 2omits illustrating a support to be used for supporting the condenser tothe distillation column). Though the connecting member is notparticularly limited on account of shape (such factors as radius ofcurvature, angle of bend, and angle of diversion), it is preferred toshorten the distance of connection with a view to heightening theeffect. To be specific, it is proper to connect the distillation columnand the condenser with a 90° (miter)bend/elbow joint or a shortpipe/reducer. The connection of the connecting line to the distillationcolumn or to the condenser can be attained by any of known methods suchas welding or bolt-and-but fastening through flanges. The easilypolymerizable matter-containing substance F such as (meth)acrylic acidcontaining impurities is supplied to the distillation column 1 andsubjected therein to distillation of (meth) acrylic acid. Thepolymerization of the vapor of the easily polymerizable matter such as(meth)acrylic acid in the connecting line 9 can be allayed because thelength of the connecting line 9 from the top of the distillation column1 to the condenser 5 is shorter than heretofore usual. Naturally, it isnecessary that the linear velocity of the vapor in the connecting line 9be not less than 5 m per second and the retention time thereof not morethan 3 seconds. The liquid as of (meth)acrylic acid cooled indirectlywith a known coolant such as water or cooling liquid in the condensers 5is handled as a reflux liquid R or a distillate D.

[0046] The re-boiler is disposed directly in the lower part of theinterior of the distillation column 1. When the distillation column is aplate column, it is fated to be positioned below the lowest stage in theplate column. The re-boiler mentioned above which is directly disposedin the lower part of the distillation column 1 is preferred to becomposed of a cylindrical vessel 8 provided on the lateral side thereofwith an inlet 14 and an outlet 15 for a heating medium and tightlysealed in the upper and lower parts thereof, a plurality of heattransfer tubes or heating tubes 11 disposed between the upper and lowersides of the cylindrical vessel 8 and provided respectively therein witha vertically piercing hole, a vapor guide 12 disposed above thecylindrical vessel 8 and having a vapor vent 17 opened into the vaporphase part of the distillation column, a loose lid 16 disposed on orabove the vapor vent 17, and a down-take 13 disposed in such a manneroutside the heating tube 11 as to join the upper and lower parts of thecylindrical vessel 8.

[0047] The easily polymerizable matter-containing substance F such as(meth)acrylic acid containing impurities is supplied to the distillationcolumn 1 and subjected therein to distillation of (meth)acrylic acid.The liquid containing (meth)acrylic acid from the bottom of thedistillation column 1 is supplied to the heating tube 11 of there-boiler. Here, the liquid passes from the liquid surface positionedabove the upper re-boiler tube sheet of the re-boiler through thedown-take 13, ascends-the interior of the heating tube 11 in a boilingstate, and succumbs to natural circulation. By allowing the vapor guide12 to open into the vapor phase part of the distillation column, it ispossible not only to shorten the vaporization zone so as to relax thepolymerization within the tube but also to increase the amount of theliquid to be circulated, shorten the retention time of the liquid,prevent partial rise of the temperature of (meth)acrylic acid, andpreclude the polymerization of the polymerizable matter. The vaporformed by the boiling ascends, passes through the vapor guide 12, entersthe distillation column 1, and succumbs to the ordinary treatment ofdistillation. The outlet of the heating tube 11 of the re-boilergenerally constitutes itself the phase of a vapor-liquid mixture. Byproviding the loose lid 16 and the vapor guide 12, it is possible toprevent the liquid from the distillation column from directly enteringthe heating tube 11 and allay or abolish the resistance to the passageof the liquid hold-up on the upper surface of the heating tube 11 of there-boiler. The heating for the boiling mentioned above is effected byintroducing a heat medium such as steam through a heat medium inlet 14and the heat medium spent in exchange of heat is discharged through anoutlet 15. Then, the bottom stream is discharged through an outlet 19(B). Since the re-boiler is disposed directly in the distillation column1 and further since it is provided with the vapor guide 12 adapted toconverge the vapor arising from the heating tube 11 of the re-boiler,the linear velocity of the vapor can be set at a level of not less than5 m per second and the retention time of the vapor at a level of notmore than 3 seconds and the polymerization of the vapor of the easilypolymerizable matter such as (meth)acrylic acid between the re-boilerand the distillation column 1 can be allayed. In addition, referencenumeral 20 is a plate with a plurality of fine holes, reference numeral21 an inlet of a coolant such as water, and reference numeral 22 anoutlet thereof.

[0048]FIG. 3 is a fragmentary sectional view of another embodiment ofthe connecting line between the distillation column and the condenser inFIG. 2. In FIG. 3, a (inter) connecting line 39 such as a miter bend isequipped between the distillation column 31 with the plate 20 and thecondenser 35 with an outlet 32 of a coolant by means of bolt-and-nutfastening through flanges. The inner diameter of the connecting line 39is substantially identical to that of shell of the condenser 35 from thecolumn 31 to the tube sheet 34 of the condenser 35 in the view point ofincreasing good distribution of the vapor and preventing the channelingthereof.

[0049]FIG. 4 is a fragmentary front view of another embodiment of pipesbetween the condenser 45 and the R place of the distillation column inFIG. 2. In FIG. 4, a pump 40 is equipped between the pipe by means ofbolt-and-nut fastening through flanges.

[0050]FIG. 5 is a fragmentary front view of another embodiment of thedown-take in FIG. 2. In FIG. 5, another embodiment of a down-take 53 isequipped between the distillation column with a re-boiler 58, which hasan inlet 54 and outlet 55, of a heat medium, and the pipe 52 by means ofbolt-and-nut fastening through flanges.

[0051] In the manner described above, the polymerization of the easilypolymerizable matter-containing substance such as (meth)acrylic acid inthe connecting line can be effectively prevented.

[0052]FIG. 6 is a sectional view of a distillation column such inconstruction that a condenser is directly disposed in the top of thedistillation column and a re-boiler is attached to the lateral lowerpart of the distillation column, both condenser and re-boiler having aplurality of tubes therein. Here, the condenser and the distillationcolumn are not particularly limited but may use well-known devices suchas a vertical heat exchanger and a plate column. The connection of thedistillation column and the condenser can be attained by well-knownmethods such as welding or bolt-and-nut fastening through flanges. Inthe present diagram, like members or parts found in FIG. 1 arerepresented by like reference numerals. With reference to FIG. 6, Thevertical shell-and-tube heat exchanger 5 is directly disposed in the toppart 3 of the distillation column 1. The easily polymerizablematter-containing substance F such as (meth)acrylic acid containingimpurities is supplied to the distillation column 1 and subjectedtherein to distillation of (meth)acrylic acid. In the distillationcolumn 1, the vapor of the easily polymerizable matter such as (meth)acrylic acid enters the condenser 5. The vapor of the easilypolymerizable matter that has entered into the condenser 5 is condensedas indirectly cooled with a well-known coolant such as water or acooling liquid. The liquid resulting from the condensation is receivedin a pan 4 disposed in the upper part of the distillation column 1,recovered as a reflux liquid R in the distillation column 1, and furtherprocessed as occasion demands (D). A plate with a plurality of fineholes may be provided between the pan 4 and the condenser 5 in order toavoid the liquid condensed to directly enter the column 1 through theopening of the pan 4. The plate may be provided among the inner wallsbut provided in order to only cover the opening of the pan 4. Since thecondenser 5 is directly disposed in the top part 3 of the distillationcolumn 1 and the linear velocity of the vapor is set at a level of notless than 5 m per second and the retention time at a level of not morethan 3 second as required, the polymerization of the vapor of the easilypolymerizable matter such as (meth)acrylic acid between the distillationcolumn and the condenser 5 can be allayed.

[0053] Further, the re-boiler 7 is disposed in the lower lateral part ofthe distillation column 1 using a (inter) connecting line. Here, there-boiler is not particularly limited but may use any of the deviceswell known to the art. The vapor phase volume of the connecting line 6is decreased to the fullest possible extent by minimizing the distancebetween the outlet of the re-boiler 7 and the inlet of the distillationcolumn 1. For this purpose, the outlet of the re-boiler 7 is connectedto the straight shell near the bottom of the distillation column 1 (FIG.6 omits illustrating a support for fastening the re-boiler to thedistillation column). Though the connecting member or pipe is notparticularly limited on account of shape (such factors as radius ofcurvature, angle of bend, and angle of diversion), it is preferred toshorten the distance of connection with a view to heightening theeffect. To be specific, it is proper to connect the distillation column1 and the condenser 5 with a 90° bend/elbow joint or a shortpipe/reducer. The connection of the connecting line 6 to thedistillation column 1 or to the re-boiler 7 can be effected by suchwell-known methods as welding or bolt-and-nut fastening through flanges.The liquid of the easily polymerizable matter such as (meth) acrylicacid in the bottom part of the distillation column 1 passes through theconnecting line 2 and enters the re-boiler 7. The vapor of the liquidformed by re-boiling in the re-boiler 7 passes through the connectingline 6 and enters the vapor phase part of the distillation column 1. Inthe connecting line 6, the polymerization of the vapor is prevented orallayed because the linear velocity of the vapor is set at a level ofnot less than 5 m per second and the retention time thereof at a levelof not more than 3 seconds. The bottom stream in the re-boiler 7 isdischarged via the connecting line 2 (B).

[0054] In addition, reference numeral 20 is a plate with a plurality offine holes, reference numeral 14 an inlet of a heat medium, referencenumeral 15 an outlet thereof, reference numeral 21 an inlet of a coolantsuch as water, and reference numeral 22 an outlet thereof.

[0055]FIG. 7 is a fragmentary sectional view of another embodiment of a(inter)connecting line between the distillation column and the re-boilerin FIG. 6. In FIG. 7, a (inter)connecting line 76 such as a miter bendis equipped between the distillation column 71 with the plate 20 and there-boiler 77 with an inlet 74 of a heat medium by means of bolt-and-nutfastening through flanges. The inner diameter of the connecting line 76is substantially identical to that of shell of the re-boiler 77 from thecolumn 71 to the tube sheet 78 of the re-boiler 77 in the view point ofgood distribution of the vapor and preventing the channeling thereof.

[0056] The polymerization of the easily polymerizable matter-containingsubstance such as (meth) acrylic acid in the connecting line can beeffectively prevented as described above. In a similar manner as above,the polymerization of (meth)acrylic acid ester can be effectivelyprevented.

[0057] The case of having a condenser disposed in the straight shell ofthe upper part of a distillation column and a re-boiler disposed in thestraight shell of the lower part inside the distillation column, thecase of having a condenser disposed in the straight shell of the upperpart of a distillation column and a re-boiler disposed directly in thelower part of the distillation column, and the case of having acondenser disposed directly in the top of a distillation column and are-boiler disposed in the straight shell of the lower part of thedistillation column have been illustrated. This invention does not needto be limited to these cases. The condensers and the re-boilers may beused either singly or in an arbitrary combination.

[0058] By the construction described above, it is possible to reduce thedistance of passage of the vapor between the distillation column and theheat exchanger substantially to zero or to set the linear velocity ofthe vapor in the connecting line intervening between the distillationcolumn and the heat exchanger at a level of not less than 5 m per secondand the retention time thereof at a level of not more than 3 seconds.Thus, even when the distillation of a liquid mixture containing acrylicacid, acetic acid, propionic acid, acrolein, maleic acid and/or formalinis continued for a long time, virtually no sign of the adhesion of apolymer to the part of the connecting line exposed to the liquid mixturedue to the effect of the retention time is recognized.

EXAMPLES

[0059] Now, this invention will be described more specifically belowwith reference to examples. However, this invention is not limited bythe following examples.

Example 1

[0060] Acrylic acid was refined by the use of a distillation columnmeasuring 1100 mm in inside diameter and having 50 sieve trays ofstainless steel (SUS316) built therein. The distillation was performedcontinuously for one week by feeding a feed liquid composed of 98 wt. %of acrylic acid and 2 wt. % of acetic acid under the conditions of 53.3hPa-abs (40 mmHg) of column top pressure, 63° C. of column toptemperature, 120 hpa-abs (90 mmHg) of column bottom pressure, 84° C. ofcolumn bottom temperature, and 7 of reflux ratio. (At the end of thecontinuous operation, the acrylic acid in the bottom had an assay of99.95%.) A condenser (vertical shell-and-tube heat exchanger) wasconnected to the distillation column by means of a vapor line andsupported to the column by means of a structure (not shown) in thestraight shell near the top of the distillation column and a flangedconnecting pipe. In this case, the linear velocity of the vapor in thevapor line was 20 m/s and the retention time thereof was 0.1 second.(The retention time was determined by the formula mentioned above;Retention time=G/(H×I)=2.1/(20×1.1)=0.1.) The pressure loss between thetop of the column to the condenser was constant during the course ofdistillation. When the distillation column was opened and the interiorthereof was visually inspected, the surfaces of tubes from the top ofthe column through the condenser showed no sign of adhesion of apolymer.

[0061] Further, a re-boiler was connected to the distillation column bymeans of a vapor line and supported to the column by means of astructure (not shown) in the straight shell near the bottom part of thecolumn and a flanged connecting pipe. Here, the vapor line opened intothe vapor phase part of the distillation column. In the vapor line, thelinear velocity of the vapor was about 40 m/s and the retention timethereof was 0.1 second (G/(H×I)=1.2/(40×0.3)=0.1). When the column wasopened and the interior thereof was visually examined, the vapor line ofthe re-boiler showed no sign of adhesion of a polymer.

Comparative Example 1

[0062] The distillation was carried out by following the procedure ofExample 1 while extracting the column top vapor through the top of thecolumn and using the condenser mounted on a structure (retention time ofthe column top vapor 3.5 seconds) and the re-boiler set on a structure(retention time of the vapor 3.2 seconds). (These retention times weredetermined by the formula mentioned above; Retention time incondenser=G/(H×I)=77/(20×1.1)=3.5 and retention time inre-boiler=38/(40×0.3)=3.2.).

[0063] During the course of the distillation, the pressure loss from thecolumn top through the condenser reached 2 mmHg in one week. When thecolumn was opened after the stop of the operation and the interiorthereof was visually examined, the occurrence of about 100 liters ofpolymer in the connecting pipe from the column top to the tube sheet ofthe condenser was confirmed. In the bottom of the column, no sign ofrise of pressure loss was recognized. When the column was opened and theinterior was visually examined, the occurrence of about 130 liters ofpolymer on the vapor line of the re-boiler was confirmed.

Example 2

[0064] (Case of Having a Re-Boiler Disposed Directly in the Lower partInside a Distillation Column)

[0065] Acrylic acid was refined by the use of a distillation columnmeasuring 1400 mm in inside diameter and having 10 sieve trays ofstainless steel built therein. The distillation was performedcontinuously for three weeks by feeding a feed liquid composed of 99% ofacrylic acid and 1% of impurities including the dimer or acrylic acidunder the conditions of 46.7 hpa-abs (35 mmHg) of column top pressure,63° C. of column top temperature, and 0.3 of ref lux ratio. (At the endof the operation, the acrylic acid at the top thereof had an assay ofabove 99.95%).

[0066] A re-boiler was a natural circulation type one provided with adown-take outside the heating tubes and further furnished with a vaporguide opening into the vapor phase part of the column and disposeddirectly in the distillation column. When the column was opened and theinterior thereof was visually examined, no sign of adhesion of a polymerwas recognized.

Example 3

[0067] (Case of Having a Condenser Directly Disposed in the Top of aDistillation Column)

[0068] The distillation was carried out by following the procedure ofExample 1 while having a condenser disposed in the top of a distillationcolumn through flanges. The distillation was continued for two weeks.(At the end of the operation, acrylic acid at the bottom thereof had anassay of 99.95%). When the column was opened and the interior thereofwas visually examined, the top of the column and the condenser showed nosign of adhesion of a polymer.

Example 4

[0069] 2-Ethylhexyl acrylate was refined by the use of a distillationcolumn measuring 1200 mm in inside diameter and having 20 sieve trays ofstainless steel built therein.

[0070] The distillation was performed continuously for three weeks byfeeding a feed liquid composed of 98.5 wt. % of 2-ethylhexyl acrylateand 1.5 wt. % of an impurity having a high boiling point under theconditions of 26.7 hpa-abs (20 mmHg) of column top pressure, 107° C. ofcolumn top temperature, and 0.4 of reflux ratio. A condenser (verticalshell-and-tube heat exchanger) was connected to the distillation columnby means of a vapor line and supported to the column by means of astructure (not shown), the inner diameter thereof was identical to thatof the condenser, in the straight shell near the top of the distillationcolumn and a flanged connecting pipe. In this case, the linear velocityof the vapor in the vapor line was 22 m/s and the retention time thereofwas 0.09 second. (The retention time was determined by the formulamentioned above; Retention time=G/(H×I)=0.39/(22×0.2)=0.09.) Thepressure loss between the top of the column to the condenser wasconstant during the course of distillation. When the distillation columnwas opened and the interior thereof was visually inspected, the surfacesof tubes from the top of the column through the condenser showed no signof adhesion of a polymer.

[0071] Further, a re-boiler was connected to the distillation column bymeans of a vapor line and supported to the column by means of astructure (not shown) in the straight shell near the bottom part of thecolumn and a flanged connecting pipe in which the inner diameter of thevapor line was identical to that of the re-boiler. Here, the vapor lineopened into the vapor phase part of the distillation column. In thevapor line, the linear velocity of the vapor was about 11 m/s and theretention time thereof was 0.06 second (G/(H×I)=0.11/(11×0.16)=0.06).When the column was opened and the interior thereof was visuallyexamined, the vapor line of the re-boiler showed no sign of adhesion ofa polymer.

[0072] The entire disclosure of Japanese Patent Application No. 11-97893filed on Apr. 5, 1999 including specification, claims, drawings andsummary are incorporated herein by reference in its entirety.

What is claimed is:
 1. A method for refining or separating an easilypolymerizable matter-containing substance by the use of a distillationcolumn equipped with a heat exchanger, which method comprises: setting alinear velocity of the vapor in at least one connecting line between thedistillation column and the heat exchanger at a level of not less than 5m per second and a retention time of the vapor in the (inter) connectingline at a level of not more than 3 seconds excluding zero.
 2. A methodaccording to claim 1, wherein the linear velocity is in the range of 5to 60 m/s.
 3. A method according to claim 2, wherein the linear velocityis in the range of 20 to 60 m/s.
 4. A method according to claim 1,wherein the retention time is at a level of not more than 2 secondsexcluding zero.
 5. A method according to claim 1, wherein the heatexchanger is a vertical shell-and-tube heat exchanger and is supportedto the distillation column.
 6. A method according to claim 5, whereinthe connecting line for the vapor connected to the heat exchanger isdisposed to the straight shell part of the distillation column.
 7. Amethod according to claim 1, wherein the heat exchanger comprises atleast two items of a condenser and a re-boiler.
 8. A method according toclaim 1, wherein the heat exchanger is a condenser and is disposeddirectly in the top part of the distillation column.
 9. A methodaccording to claim 7, wherein the heat exchanger is a re-boiler and there-boiler being disposed directly inside the distillation column in thelower part thereof wherein a vapor guide provided above the heatingtubes for the re-boiler is opened into the vapor phase part of thecolumn and a down-take is furnished outside the heating tubes.
 10. Amethod according to claim 1, wherein an inner diameter, against the flowof the vapor, of the connecting line substantially coincides to that ofthe shell of the heat exchanger.
 11. A method according to claim 1,wherein the easily polymerizable matter is a vinyl compound.
 12. Amethod according to claim 11, wherein the vinyl compound is(meth)acrylic acid and/or a (meth)acrylic ester.
 13. An apparatus forrefining an easily polymerizable matter-containing substance comprising:a distillation column, and a heat exchanger disposed directly in the toppart of the distillation column, wherein the heat exchanger serves as acondenser.
 14. An apparatus according to claim 13 further comprising are-boiler, wherein a (inter)connecting line for the vapor connected tothe re-boiler is connected to the lower lateral part of the distillationcolumn.
 15. An apparatus for refining an easily polymerizablematter-containing substance comprising: a distillation column, and aheat exchanger disposed directly in the lower part inside thedistillation column, in which a vapor guide provided above the heatingtubes for the exchanger is opened into the vapor phase part of thecolumn and a down-take is furnished outside the heating tubes, whereinthe heat exchanger serving as a re-boiler.
 16. An apparatus according toclaim 15, wherein the re-boiler comprises: a cylindrical vessel providedon the lateral side thereof with an inlet and an outlet for a heatingmedium and tightly sealed in the upper and lower part thereof, aplurality of heat transfer tubes disposed between the upper and lowersides of the cylindrical vessel and provided respectively therein with avertically piercing hole, a vapor guide disposed above the cylindricalvessel and having a vapor vent positioned approximately in the centralpart of the distillation column, a loose lid disposed on or above thevapor vent, and a down-take disposed in such a manner outside theheating tube as to join the upper and lower parts of the cylindricalvessel.
 17. An apparatus according to claim 14 further comprising acondenser, wherein a (inter)connecting line for the vapor connected tothe condenser is connected to the upper straight shell of thedistillation column.